Metal halide discharge lamps are favored for their high efficacies and high color rendering properties which result from the complex emission spectra generated by their rare-earth chemistries. Particularly desirable are low-wattage ceramic metal halide lamps which offer improved color rendering, color temperature, and efficacy over traditional quartz arc tube types. This is because ceramic arc tubes can operate at higher temperatures than their quartz counterparts and are less prone to react with the various metal halide chemistries. Like most metal halide lamps, ceramic lamps are typically designed to emit white light. This requires that the x,y color coordinates of the target emission lay on or near the blackbody radiator curve. Not only must the fill chemistry of the lamp be adjusted to achieve the targeted emission, but this must also be done while maintaining a high color rendering index (CRI) and high efficacy (lumens/watt, LPW).
Most commercial ceramic metal halide lamps contain a complex combination of metal halides, particularly iodides. In general, iodides are more favored than fluorides because of their lower reactivity and are more favored than chlorides or bromides because they tend to be less stable at higher temperatures. Thallium iodide is a common component which is mainly used to adjust the (x,y) color coordinates so that they lay on the blackbody curve. For example, a commercial 4200 K lamp may contain mercury plus a mixture of TlI, NaI, DyI3, HoI3, TmI3, and CaI2. While lamps that contain thallium operate well at their rated power, their photometric characteristics deteriorate when the lamps are dimmed. This is primarily because the vapor pressure of thallium iodide is much higher than the vapor pressures of the other fill components. As the lamp power is reduced, the operating temperature of the arc tube is lowered and the 535 nm thallium atomic emission line begins to dominate the emission spectrum of the lamp. The disproportionate increase in the thallium emission causes the lamps to attain higher color temperatures and shifts the x,y color coordinates significantly above the blackbody curve. As a result, the dimmed lamps acquire an undesirable greenish hue. Experiments have shown that the higher the percentage of thallium in the fill, the greater the green shift.
Another problem with thallium-containing fills is that small temperature variations (±50° C.) lead to large variations in the correlated color temperature (CCT). This is problematic because the fill chemistry must be re-optimized each time a new outer jacket or reflector is added even though the arc tube and desired color coordinates are identical. Thallium iodide also has been associated with a low power factor (PF) and higher re-ignition (RI) peaks in some metal halide lamps. A low power factor means a less efficient lamp-ballast system and large RI peaks can cause excessive wall blackening. And lastly, thallium has been prohibited from use in U.S. household products since 1975.